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Abstract

In the southeast Christchurch suburbs of Woolston and Heathcote Valley, industrial water abstraction has lowered groundwater levels in the Riccarton Gravel aquifer below mean sea level. This has resulted in the potential for surface saltwater to migrate down into Riccarton Gravel aquifer. Shallow bedrock in this area may act as a barrier to groundwater recharge, exacerbating the low groundwater levels.

Geophysical methods were used to map saltwater presence and bedrock topography in the areas of Woolston, Heathcote Valley, and Brighton Spit.

The TEM and resistivity data in the Heathcote Valley and Woolston areas suggest that downward migration of saltwater is occurring primarily through the Heathcote River, particularly the river mouth marsh area, and secondarily from the Avon-Heathcote estuary. The sources of the saltwater are therefore likely to be seawater with additional contributions from Ferry Rd landfill site. The area of saltwater contamination is wide spread in the Heathcote Valley­ Woolston area and is predominantly present within Christchurch Formation deposits. Connate seawater is unlikely to be a major contributor to seawater presence in sediments in this area.

TEM data along Brighton Spit suggest that seawater is present in Christchurch Formation deposits. The source of seawater may be connate, however the unconfined nature of Christchurch Formation sediments along Brighton Spit suggests that seawater could migrate downward due to density differences between seawater and fresh groundwater (similar to the process occurring on oceanic islands).

The magnetic method was used to delineate bedrock topography in the Avon-Heathcote estuary.
The method showed a good response to changes in bedrock topography. However, due to the nature of the response, depths to bedrock were difficult to determine.
The magnetic results suggest a shallow volcanic ridge extending out from Mt Pleasant to the northern edges of the estuary. Tentative estimates of the depth suggest that the ridge could be as shallow as 75 metres in the south and 85 metres further north.
This ridge may affect groundwater flow direction in the region, and also enhance any drawdown by restricting the aquifers ability to recharge.

Bedrock depth estimates throughout the study area do not correlate well with sites of known bedrock depth. The depth is over-estimated along Chapmans Rd buried volcanic ridge and possibly under-estimated in the Heathcote Valley. However, the contour plots of depth in areas away from rapid changes in topography are likely to be indicative of the actual depths to bedrock.

The depth estimates can be improved by re-addressing the subsurface terrain model, i.e. using it in an iterative process or using different subsurface terrain models, and by taking more gravity readings at the exact sites of known bedrock depth, and averaging them.

The gravity measurements suggest that the depth to bedrock is shallower in the upper reaches of the Heathcote Valley than initially expected. This may significantly affect groundwater recharge in the Heathcote Valley area.
The measurements also suggest the presence of another buried ridge extending in a north east direction off the end of Chapmans Rd. This may significantly affect recharge from the north and hence make abstraction wells to the south (i.e. wells on Chapmans Rd) more susceptible to pumping induced drawdown.

The gravity method would be suitable for mapping bedrock topography in other areas fringing the Banks Peninsula.